Positive acceleration or negative acceleration (i.e., deceleration) of the human body occurs whenever its velocity undergoes a change, and is a consequence of human movement. Most acceleration of the human body is benign, but when the magnitude of acceleration becomes too large, as may occur with a direct blow or impact, it can be injurious, and can be especially injurious to fragile body parts such as the brain. Furthermore, different types of acceleration can lead to different types of brain injuries. High linear acceleration can cause focal injuries such as contusions and intra-cerebral or subdural hematomas. Concussions and diffuse axonal injury (DAI) are believed to result when there is a significant rotational acceleration component of the injury mechanism. Repetitive exposure to traumatic brain injury can also lead to chronic traumatic encephalopathy (“CTE”), a progressive, degenerative brain disease.
Injurious acceleration can be experienced in many situations (e.g., in an automobile accident or in a slip and fall), but some professions and activities expose their participants to a higher probability of injurious acceleration. For example, military combat personnel may be exposed to a heightened probability of blast injuries, which can cause CTE after as few as a single exposure. Athletes may also be exposed to an elevated probability of injurious acceleration caused by a direct impact to the head, face, neck, or elsewhere on the body during the athletic activity. Brain injury can result even when the impact is not delivered directly to the head since the acceleration can be transmitted through the body to the brain. Such impacts can occur as a result of accidental or intentional collisions between athletes, or between an athlete and an inanimate object (e.g., a ball, goal post, fence, or the ground). While the probability of an injurious impact is higher in contact sports, participants in non-contact sports are not immune from such injurious impact.
When sporting injuries are considered, an elevated probability of occurrence is combined with a large and growing number of participants. The United States Centers for Disease Control estimated that more than 3,800,000 sports brain injuries occur every year, and that as many as 80% of them may go undetected and/or unrecognized. It is further estimated that more than 300,000 sports brain injuries per year result in a loss of consciousness. Additionally, individuals exposed to an elevated probability of serious head impact are also frequently exposed to high environmental temperatures due to, for example, padding and helmets for athletes, body armor for military personnel, or protective clothing for emergency response personnel. Exertion under such circumstances can lead to hyperthermia, heat stroke, and death.